The disconnect clear time is a disconnection of the A&B wires to the PBX subscriber. It is variable in 100 m/s steps from 0 to 1 seconds. The actual parameter is P16 of DSB resource. The disconnect/clear time is used by PABX's either to enable it's equipment to release or, to inform the PABX that the exchange line has recognised that the call has come to an end. There are two Problems that have come to light with regards to the disconnect/clear times, these are:
  • No dial tone. This occurs if the disconnect time is too long, the problem is that the has cleared and is ready to seize an outgoing line but System X still has the disconnect/clear condition on the line i.e. the line is disconnected hence no dial tone.
  • Phantom calls. This occurs because of the way some PABX's detect I/C calls. When the PABX has cleared and is ready to receive an I/C call it monitors the A & B legs. System X puts out a test condition onto the line after the disconnect/clear time to check whether the PBX is on or off hook. This test condition is recognised by some PABX's as an I/C call.

Because of the wide variety of PABX's in the field it is very difficult to obtain exact figures for dis/clear times. Even on the same type of PABX you can have different Software and Firmware Packages which use different dis/clear times. However as a general rule the disconnect/clear time is set to 800 m/s, but if difficulties occur either with Phantom calls or NDT this time should be reduced.

Issued by BT in 1987

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Last revised: January 28, 2023